In certain applications, it is advantageous to extrude plastic tubular products which are of superior strength and rigidity. This allows the plastic tubes to be used where the need for structural support is mandated. It is the purpose of this invention to increase the strength of tubular plastic products without resorting to expensive, exotic materials, such as liquid crystal polymers. To accomplish this purpose, this invention provides an extrusion die assembly for common thermoplastic materials which provides a rotary motion to the molten plastic material as it is extruded.
Several attempts have been made, in particular in the medical field, to increase strength and rigidity by orienting the extruded material in a helical manner. Such an attempt is illustrated in U.S. Pat. No. 5,156,785 which describes a method by which catheters are constructed from a mixture of liquid crystal polymers and other plastics. Using an extrusion die with rotating components, fibrils of liquid crystal polymer are oriented in a helical pattern in order to construct catheters with greater rotational stiffness. In U.S. Pat. No. 5,248,305, which is related to the ""785 patent, a schematic depiction of an extrusion head with rotating components is shown in FIG. 2. It should be noted that there is no description of how the relative rotation of the extrusion die elements is accomplished. It is a purpose of this invention to construct a reliable system for rotating the opposing surfaces which form an extrusion channel in an extrusion die for use with common thermoplastic materials.
It is a purpose of this invention to provide the advantages of extrusion with rotating components for applications involving larger products made from common thermoplastic materials. As the materials become less viscous, there is an increased need to avoid the dead spots of common plastic delivery systems and provide a balanced flow. It is the purpose of this invention to combine an extrusion die assembly having a balanced flow passage with rotating die elements. A balanced flow extrusion die passage is described in U.S. Pat. No. 5,667,818, the disclosure of which is incorporated herein by reference.
An extrusion die system is constructed of an assembly of axially aligned modules which are constructed having a balanced flow passage extending through the assembly to supply flowing plastic to a conical extrusion channel. The passage ends in an extended annular exit channel formed by surfaces that rotate relative to each other. The assembly consists of a die body, a die holder, and a die which are all axially aligned and constructed with an overall axial bore formed by the cooperation of the modules. A tip module is positioned in the axial bore. The tip module consists of upstream and down stream parts rotatably mounted in a tip holder. The die and the tip module cooperate to form an extended exit channel which communicates with the extrusion channel. The die is rotatably mounted on the die holder and the tip portions are rotatably mounted within the axial bore by the tip holder. Each of the rotatably mounted components is independently driven by motors. The motors are controlled by a computer to provide an infinite variation in relative rotational motion between the rotating components.
A unique bearing and seal arrangement is employed for reliable rotation of the rotating surfaces which form the extended exit channel. The bearing group at the downstream end consists of a pair of spaced radial bearings and a thrust bearing mounted in between. To minimize leakage at the interfaces of the rotating surfaces, a labyrinth type seal is constructed at the interface to create limited tortuous path for leakage which absorbs the majority of pressure drop. The labyrinth seal is immediately followed by a face seal which accommodates the remaining pressure drop. A secondary face seal is constructed adjacent in the leakage path to the primary face seal and provides an axial force which preloads the thrust bearing. The secondary face seal also provides a backup to the primary face seal in the event of a failure. A sensing passage communicates with the chamber of the secondary face seal to allow the sensing of increased pressure to provide a warning of primary seal failure.
The bearings are situated in a closed oil bath which utilizes a high viscosity oil. The bath consists of the bearing compartments and a chamber into which fins extend. The fins are attached to a rotating component and operate to circulate oil as the components rotate.